Prodigal son immersed? (6)

I believe the answer is:

waster

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'prodigal son' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for waster that I've seen before include "Ne'er do well, profligate" , "Ne'er-do-well, spendthrift" , "Layabout" , "Useless person, good-for-nothing" , "Idler, good-for-nothing" .)

I've seen this clue in The Guardian.

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